1. Field of Invention
The present invention relates in general to monitoring techniques for the Gas Tungsten Arc Welding process and more particularly to detecting whether the work piece has been fully penetrated.
2. Description of Related Art
Gas tungsten arc welding (GTAW) has become an indispensable tool for many industries because of the high-quality welds produced and low equipment costs[1]. Mechanized/automated systems rely on precision control of joint fit-up and welding conditions to use the pre-programmed welding parameters to produce repeatable results. However, precision control of joints and welding conditions is very costly and not always guaranteed. Up to date, there are no satisfactory sensors/approaches that can be conveniently carried by the torch to automatically monitor the penetration depth (how far the liquid metal penetrates along the thickness of the base metal) or the degree of the full penetration like a skilled welder.
The difficulty is primarily due to the invisibility of the liquid metal bottom surface underneath the weld pool and the extreme brightness of the arc and various methods have been studied, including pool oscillation, ultrasound, infrared sensor, etc. The pioneering work in pool oscillation was conducted by Kotecki[2], Richardson[3], Hardt[4] and their co-workers. Den Ouden found an abrupt change in the oscillation frequency of the pool during the transition from partial to full penetration[5, 6]. At Georgia Institute of Technology, Ume leads the development of non-contact ultrasonic penetration sensors based on laser-phased array techniques[7, 8]. Because the temperature distribution in the weld zone contains abundant information about the welding process, infrared sensing of welding processes has been explored by Chin at Auburn University [9-12]. The penetration depth has been correlated with the infrared characteristics of the infrared image. At MIT, Hardt used an infrared camera to view the temperature field from the back-side[13]. The penetration depth was precisely estimated from the measured temperature distribution and then controlled[14]. Because of the difficulty of the problem and the urgency for solution, researchers around the world have continued the explorations [15-19].
Zhang found that the average sag depression of the solidified weld bead has a good linear correlation with the back-side bead width[20, 21]. A structured-light vision sensor and image processing algorithm were thus developed to measure the sag geometry in GTAW. By modeling the arc welding process, an adaptive control system has been completed to achieve the desired back-side bead width[22]. Recently, the University of Kentucky developed an innovative method to measure the 3D geometry of the weld pool surface for both GTAW and gas metal arc welding (GMAW)[22, 23]. It projects a low power laser pattern onto the mirror/specular surface of the weld pool. The laser pattern reflected from the weld pool surface remains the laser intensity when travelling from the arc and weld pool but the arc radiation loses its intensity. The reflected laser and arc radiation can thus be intercepted and be imaged on the interception plane. Because the arc radiation reduces as the travel distance increases, the reflected laser pattern (signal) can be clearly distinguished from the arc radiation (background). The 3D weld pool surface that reflects the laser pattern can then be computed from the measured laser reflection pattern and its known incident pattern based on the law of reflection. A vision system may thus be developed to emulate a skilled welder to observe and control the weld joint penetration. However, easily measurable arc signals, arc voltage and arc current, may be more durable and cost-effective and thus more suitable for industrial applications.
Possible relationship between weld joint penetration and arc signals has been extensively studied at the University of Kentucky Welding Research Lab[24-27]. Successful monitoring and control over the weld joint penetration have been achieved for plasma arc welding (PAW) process. Since PAW is an extension of GTAW process with a constrained arc for higher energy and heat density, it is ideal if the method for PAW process control can be extended to GTAW. Unfortunately, the inventors of this invention found that for the unconstrained free arc in GTAW, the arc voltage does not increase as the weld penetration increases as in PAW and as one may expect. Instead, the arc voltage decreases first as the weld penetration increases. It increases only after the full penetration is sufficiently established. An innovative arc signal based weld penetration monitoring and control method is thus invented based on this characteristic of the arc voltage change as the weld pool surface develops in GTAW and is used to solve the weld penetration control issue in GTAW pipe welding.